1. Technical Field
The present invention relates to a piezoelectric resonator element and a piezoelectric resonator.
2. Related Art
AT-cut piezoelectric resonator elements use a thickness-shear vibration mode which is ideal for obtaining excellent temperature properties as their primary vibration mode. Such an AT-cut resonator element is processed by mechanical processing or photolithography.
JP-A-58-47316 discloses a piezoelectric resonator that vibrates in a so-called mesa-type thickness-shear vibration mode where an energy-trapping effect equivalent to a bevel or convex structure is obtained.
In recent years, there is a strong tendency to decrease the size of resonator elements, and there is a strong demand for a resonator having a small side ratio. In general, a resonator having a small side ratio is likely to be affected by a mode (contour vibration mode) associated with the contour of the resonator, and a thickness-shear mode is suppressed by being coupled to the contour vibration mode.
JP-UM-A-6-52230 discloses a mesa-type AT-cut quartz crystal resonator which is designed so as to solve a problem where in the boundary between a mesa portion and a thin peripheral portion, if the lateral walls of the boundary portion are at 90° to the primary surface, extraction electrodes (lead electrodes) extended from excitation electrodes are short-circuited. In the AT-cut quartz crystal resonator, the lateral walls of the boundary portion are configured as sloped or curved surfaces so as to prevent short-circuiting of the lead electrodes.
JP-A-2001-230655 discloses a mesa-type AT-cut quartz crystal resonator in which coupling of a thickness-shear vibration mode which is a primary vibration mode and a flexural vibration mode is suppressed by configuring the lateral walls of the boundary between a mesa portion and a thin peripheral portion so as to be sloped at 63° and 35° rather than being vertical (90°).
Japanese Patent No. 4341583 discloses a structure in which coupling of a thickness-shear mode and a flexural mode is suppressed by appropriately setting the mesa dimensions and the distance between mesa electrodes which are associated with the flexural vibration mode which is a contour vibration mode.
JP-A-2008-263387 discloses a mesa-type AT-cut quartz crystal resonator in which a spurious mode is suppressed by appropriately setting the amount of etching of a step.
JP-A-2010-62723 discloses a mesa-type AT-cut quartz crystal resonator in which a spurious mode is suppressed by appropriately setting the short side length of a piezoelectric substrate and the short side length of a vibrating portion.
JP-A-2-57009 discloses a structure in which vibration energy in primary vibration mode is effectively trapped by forming a mesa portion with multiple steps.
Japanese Patent No. 3731348 discloses a resonator of which the cross section has a staircase shape, which can be formed by chemical processing such as etching or mechanical processing such as sand blast while gradually changing the dimensions of a resist covering the resonator portion during manufacturing.
JP-A-2008-236439 discloses a structure in which the step between a thick central portion and a thin peripheral portion is formed in a staircase shape with plural steps, whereby it is easy to form the shape of a resist pattern, and to deposit an electrode material to a sufficient thickness, and make the thick central portion have a convex shape, thus enhancing an energy trapping effect.
JP-A-2009-130543 discloses a multi-step mesa-type resonator element in which entry of a conductive adhesive agent into a mesa portion is suppressed by a step which is used as a stopper for the adhesive agent.
JP-A-2010-109527 discloses a multi-step mesa structure which is precisely formed by etching a base material made of a quartz crystal using photolithography.
Japanese Patent No. 4075893 discloses a method of manufacturing a multi-step mesa-type AT-cut quartz crystal resonator in which a mesa structure is formed using a laser.
As described above, JP-A-2-57009, Japanese Patent No. 3731348, JP-A-2008-236439, JP-A-2009-130543, JP-A-2010-109527, and Japanese Patent No. 4075893 disclose a multi-step mesa structure in which the number of mesa steps is increased as a structure for suppressing coupling with a flexural mode through a strong energy trapping effect.
However, in a multi-step mesa-type thickness-shear-mode piezoelectric resonator element in which the extending direction of the longer side is parallel to the X axis, particularly in a piezoelectric resonator element in which the X-side ratio is small, it is obvious that a thickness-shear vibration mode couples to a spurious mode such as a contour vibration mode, and the CI (Crystal Impedance) value increases.